Substantial research suggests that the prevalence of persistent pain and sensitivity to certain laboratory pain stimuli increase with advancing age. These effects may be due to a decline in the efficacy of pain-inhibition in older adults. The goal of the proposed research is to characterize potential age-associated decrements in endogenous pain modulation. The specific aims of this project are: 1) to investigate the existence and magnitude of such decrements; 2) to examine potential physiological mechanisms; 3) to determine their clinical relevance; and 4) to elucidate the role of endogenous opioids in pain inhibition. Eighty subjects (40 younger, 40 older) will undergo assessment of diffuse noxious inhibitory controls (DNIC), a phenomenon which refers to the decrease in perceived pain from a noxious stimulus produced by concurrent application of a second noxious stimulus. Induction of DNIC is a frequently-used methodology for assessing the integrity of endogenous pain-modulatory systems in humans. DNIC will be assessed under conditions of double blind administration of naloxone (producing opioid blockade) or saline (placebo control). The proposed project will broaden an extant program of research investigating senescent alterations in pain responses and will provide important and novel information regarding age-related alterations in endogenous pain modulation.